Automatic frequency control



June 30, 1953 B.- A. TREvoR AUTOMATIC FREQUENCY CONTROL 4 Sheets-Sheet l Filed May 11, 1950 INVENTOR BezrmAfreVar BULL..." IM

ATTORN EY I mv Ev wv wm Sv ,lune 31.1, 1953 BA1-VOR 644,035

AUTOMATIC, FREQUENCY CONTROL Filled nay 11, 195o 4 sheets-sheet :s

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ATTORNEY June 30, 1953 B. A. TREVQR AUTOATI'QFREQUENCY coNTRoL 4 sheets-sheet .4

Filed May 11, 1950 mark.

Patented June 30, 1953 UNITED stares l2,644,035 AUTOMATIC FREQUENCY CONTROL Bertram A. Trevor, Riverhead, N.' Y., assig'nor to Radio Corporation of America, a corporation of Delaware sans n Application May 11, 1950, Serial No. 161,305

l0 Claims. e

This invention relates to automatic frequency control (AFC) of an oscillator. More particularly, it relates to a system for AFC of a heterodyne oscillator in a telegraphy receiver.

Generally, this invention operates to maintain proper receiver tuning when receiving on-off telegraph or frequency shift (FS) telegraph signals. Thus, the system of this invention is operative when receiving any of the following types of signals: on-oii` telegraph, ori-off printer telegraph, FS telegraph, or amplitude, phase or frequency modulated telephone.

For on-of telegraph signals, continuous AFC of the receiver should be held as long as the signal is being keyed. Correction should be independent of keying bias or per cent mark. Also, the tuning correction should cease if the signal keying stops or if the signal goes off the air. Under these conditions, the receiver tuning should remain xed for an indenite time after the signal has disappeared. If it did not, the tuner might be driven outside its automatic control range at such times.

For on-off printer signals, continuous AFC of the receiver should be held as long as the signal is being received. Correction should be independent of keying bias or per cent mark. AFC should remain in effect as long as the signal is on the air, Whether or not it is being keyed. Ori-off printer signals usually stop on the mark condition, which is equivalent to a continuous carrier. If the signal goes off the air the tuning correction should Vbe inoperative. Under these conditions, the receiver tuning should remain fixed for an indefinite time after the signal has disappeared.

For FS tele-graph signals, the center frequency between mark and space should be tuned to the center of the receiver selectivity. Also, the receiver tuning should be independent of per cent mark or keying bias. Further, such ltuning should be independent of the amount of frequency shift. In some cases the lAFC should become inoperative whether the signal stops on or space or any frequency between. In other cases the AFC should remain operative if the signal stops on If desired. a carrier operated control may be used to disable the tuning mechanism after the signal goes olf the air.

For phase, frequency or amplitude modulated telephony the tuning correction should be continuous. Also, it should become inoperative if the signal goes off the air. Under these conditions, the receiver tuning should remain ixed for an indefinite time after the signal has disappeared.

the absence of keying,`

-3 being had to the motor.

Accordingly, an object of this invention is to devise an arrangement meeting the above requirements.

More specifically, an object of this invention is to provide an AFC arrangement which'will function continuously during reception of keyed signal but will be disabled when the received frequency shift keyed (FSK) signal is not being keyed or when it goes off the air.

Another object is to maintain AFC for 01T keyedsignal when on steady mark.

A further object is to squelch the AFC to -prevent operation thereof by high receiver noise under weak signal or no signal conditions.

An additional object is to provide an improved motor control circuit for AFC. Y

The foregoing and other objects of the linvention will be best understood from the following description of some examples thereof, reference the accompanying drawings, wherein:

Fig. 1 is a schematic diagram of an AFC system according to this invention;

Fig. 2 (on two sheets) is a similar diagramof a modification; and

Fig. 3 is a schematic diagram of a modified motor control circuit.

The objects of this invention are accomplished, briefly, in the following manner:

The output voltage of a discriminator controls the iiow of alternating current to one winding of a two-phase motor, by means of an electronic circuit. The other winding of this motor is Supplied I with alternating current directly from the line. The motor drives a condenser in the hetercdyne oscill'ators circuit to tune such oscillator. In one embodiment, a relay is energized when keying stops or in the absence of signal to short-circuit the alternating current input to .one winding of This in effect disables the AFC circuit. In another embodiment, a thresholding system biases off Va tube lsupplying alternating current to the motor in response to the appearance of a weak signal or one that momentarily fades intogthe noise lever. An additional threshold circuit is provided to squelch the AFC when itv might otherwise be enabled .by the noise output of the discriminator. f

`Now referring to Fig. l, a single AFC circuit is shown.A Actually the AFC unit in a practical system would consist of three identical circuits as shown in this figure, one of which is operated iny connection with each separate receiver of a three-set diversity system when separate beating oscillators are used. In case a beating oscillator common to all three sets is used, then only one an 0n- AFC circuit would be required. For simplicity, this description will cover operation with a single receiver. The transmitted telegraph signals are picked up by an antenna and supplied to unit This unit consists of a cascaded radio frequency amplifier,V a mixer to which are also supplied oscillations from a high frequency oscillator in block 2, a rst intermediate frequency amplier, and another mixer to which are also supplied oscillations from an intermediate frequency oscillator in unit 2. The ll50-kc. output of unit is supplied to a second intermediate frequency arnplier 3. The LlO-kc. output of amplifier. 3 is supplied to a mixer 4 to which are also supplied, by means of coaxial cable 5, oscillations from a 40o-kc. oscillator to be later described. The frequency of this 40G-kc. oscillator is controlled by the AFC system of this invention. A portion of the output of mixer 4, which is to be maintained constant at 50 kc. is fed via line 6 to the AFC circuit to constitute the input signal thereof.

The rest of mixer lls output is fed via line l' to additional amplier and mixer stages and a signal utilization device. Such utilization device may be any suitable device, such as a tone keyer, telegraph printer, etc.

The `line E couples the 50 kc. input signal through a condenser S, with switch 9 in the position shown, to the grid Hi of amplier tube The switch 9 in the FSK position shown, is utilized for FS reception. For FS reception, the 50 k'c. signal is amplified by tube il. It is then applied through switch 9a (ganged with switch 3) and a coupling condenser to the grid i2 of tube I3. Tube V|3 functions as a limiter by virtue of current saturation and grid limiting in the presence of large inputs from tube H. The output of tube 3 is coupled over a tuned circuit arrangement I4 to the grid i5 of tube I5. Y

Tube i6 is a discriminator driver which drives diodes I1 and I8 connected in a Crosby or doubletuned-type discriminator circuit. The cathode of diode I8 is connected to ground. The discriminator I?, i3, etc. is driven by tube IG, the tuned circuits driving rectiers V! 'and i8. The upper `tuned circuit resonates at 48 kc, and the bottom circuit resonates at 52 kc. `With 50 kc. input to tube i6, both circuits drive the top and bottom diodes I8 with equal voltages.' This makes point D v(between the two output resistors) negative with respect to ground and point A at ground potential. A slight change in frequency will still lea-ve point D negative, but point A will gopositive or negative for a lower or higher frequency input, respectively.

For FS input signals to the discriminatory D. C. keying is delivered at point A or the cathode of diode il. If the mark and space frequencies are equally disposed about the discriminator center frequency, the keying at point A will have vequal excursions plus and minus with respect toa base value or ground. The D. C. keying at point A, is coupled through condenser i9 to the grid 251 of a cathode follower driver tube 2|. The excursions at point A are coupled from the cathode of tube 2l through condenser 22 to the cathode of a diode 23 and the anode of a diode 24. The resistor between point A and the cathode of diode 23 does not appreciably drive the diodes 23. 2li. due to the low output impedance of the cathode follower 2|. 'I'his resistor will pass a steady direct potential, held for a long time, onto the diodes 23,",24and nally to point B at the output side of such diodes. The diodes 23 and 24 are connected to function as a peak-to-peak diode rectifier or diode Voltmeter. For this purpose, a long time constant RC load, consisting of two series capacitors 25 and 23 in parallel with two series resistors 21 and 28, is connected between the anode of diode 23 and the cathode of diode 24. Capacitors 25 and 25 may be of one microfarad and resistors 21 and 28 ofk 2.2 znegohms. Since the cathode follower driver 2| functions as a low impedance source, diodes 23 and 24 serve more accurately as a peak voltmeter. It will be noted that the diodes 23 and 24 are connected in a more or less conventional voltage doubler circuit.

The excursions at point A are rectied by the peak-to-peak diode rectifier including diodes 23 and 24. Under these conditions (equal excursions with respect to ground at A), point B (between resistors 2'| and 28) will be maintained at ground potential irrespective of keying bias or per cent mark in the received signal. This is so because the peak-to-peak diode circuit 23, 24, etc. maintains equal and opposite potentials across the two capacitors 25 and 25. Each such potential is equal to the peak value of the mark andspace excursions of point A.

If the receiver tuning drifts, the mark and space frequencies at lead 5 will be unequally displaced from the discriminator center frequency. This will cause a corresponding shift in one direction, from zero, of the D. C. potential at B under keyed conditions. Likewise, a drift in the opposite direction will cause the potential at B to shift in the opposite direction. Also, for a steady carrier input to tube IE, having a frequency different from 50 kc., changes in the voltage at point A will be carried through diodes 23 and 24 to point B via the resistor joining capacitors I9 and 22 at point A.

Point B is connected to grid 29 of triode 30' midpoint of winding 33 is connected to ground' through a resistor 35. One end of the transformer primary 36 coupled to winding 33, is connected to a terminal 3l. The other end of windingr 36 is connected through a resistor 38 to a terminal 39. A resistor 4|] is connected across winding 36. Between terminals 31 and 39 is applied a suitable source of alternating voltage. In this way, the balanced triodes 3d and 3| have supplied'to their cathodes a push-pull GO-cycle voltage. Since the anodes of 30 and 3| are connected together, no (iO-cycle appears at common anode point C when the two triodes have equal D. C. grid potentials. 'I'his may be termed the balanced condition.

Point C is connected through a condenser 4| and a resistor 42 to the control grid 43 'of amplier tube 44. One winding 45 of a two-phase tuning motor 46 is connected in the plate circuit of tube 44. With proper receiver tuning, point B will be at ground or zero potential, as

previously described. If the receiver tuning drifts, the potential at B will vary from zero in oneY direction or the other, corresponding to the direction of drift. With proper receiver tuning; the mark and space frequencies at the input of discriminator l1, IB will be equally disposed about the discriminator center frequency. When the potential at B varies in one direction from the balanced condition (zero or ground potential thereat) a 60cycle voltage appears` at. point C and at the grid. 43. If the potential at B varies inthe other direction frombalance, the (iO-cycle voltage at grid 43 appears in opposite polarity, or in opposite relative phase. Tube 44 amplifles this 60-cycle voltage and. supplies it to motor winding 45. The other winding 4-1 of. motor. 46 is supplied directly from the (G-cycle line. In this. way, motor 46 is energized and driven when the potential at B varies from zero;

The network 38, 46 serves to give a total. phase shift of i- 90 to the voltage appearing at the plate of tube 44, relative to that on the GO-cycle line. This provides proper conditions for running the two-phase motor 46.

Tubes 48 and 49 are connected to serve asA a G-kc. oscillator. These tubes are connected. in a cathode-coupled type oscillator circuit. The frequency of this oscillator is controlled by tuning motor 46 through the agency of a variable condenser 5i! connected into the oscillatory circuit of such oscillator and mechanically coupled to the motor shaft through suitacblegearing; The oscillator frequency is varied whenever motor 46 is energized.

Tube 5l is connected as' a cathode follower having its grid 52 coupled tothe oscillator output. This tube supplies oscillator output thro-ugh condenser 53 to the coaxial cable 5 previously referred to, which feeds 40G-kc. heterodyning energy to mixer 4 to heterodyne the 45u-kc. energy therein to a frequency of kc;

The foregoing description has described operation of the AFC' system under FS keying conditions. Normal tuning for FS keying requires that the mark and space frequencies be equally spaced aboutv 50 kc. To accomplish this result, a disabling circuit for the AFCA system is required, the disabling coming into effect in the absence of keying, whether the signal stops on mark or space. Without such disabling, the' AFC' circuit would tune either the mark or space signal to exactly 50 kc. Such action cannot-tbe permitted. Such a disabling circuit will now be described.

Under FS keying conditions, the D. C'. keying at point A. is fed through a switch 9b (ganged with switch 9) and through coupling condensers 54 and 55 to the grid 56 of a cathode follower driver tube 57. The excursions at point A are coupledV from the cathode of tube 5T through con-l denser 56 to the cath-ode of a diode 59 and theV anode ofa diode 66;

The high resistance between the upper plates of condenser-s and 53' cannot' drive diodes 59, B due to the low output impedance. of the cath- 0de follower 51. The. diodes 5.9 and 60 are connected quite similarly to ydiodes 23 and 24 previously described, to function asY a peak voltmeter or voltage doubling rectifier. ever, the second condenser is capacitor 58, connected lbetween the output of the low impedance drive tube 5l and the anode of diode 69 and cathode of diode 59. connected to a. low impedance source of +2 volts and henceis not driven by alternating current. A capacitor 6| is connected from the anode of diode 59 to ground, and a potentiometric resistor 62 is connected from the anode of diode 59 tothecathode of diode 66. Cathode follower driverA 5F!v functions as a low impedance source driving rectiers 59 and 66, making them more accuratee ly a peak voltmeter or voltage doubler.

Under FSK conditions, the D. C. keying at A is fed to rectifiers 59, 60, developing a negative potential at theVv movable tap 63 on resistor. 62.

Here, how-V The cathode of diode' 60 isr conducting, thus energizing winding 61 Tap 63 is adjusted along resistor 62 to obtain the desired negative potential. This negativepotential at tap 63 is applied through resistor 64 to the grid 65 of a. triode 66. Such negative potential keeps tube 66 in a nonconducting condition. The windingv 61 of a relay designated generally by 58 is in series in the plate circuit of tube 65. In the presence of keying, rectiers 59, produce a negative voltage at grid of sufficient magnitude to cause tube 66 to be nonconducting,` releasing the. relay to the position shown. In such position, two circuits are opened. The long time constant of the 5-megohm resistor 62 and 0.1 microfarad capacitor 6| in the output of rectiers 59, 6D 'maintains this negative voltage nearly constant at grid 65 during keying. This keeps relay 68 in the unoperated condition.

If keying stops, a steady D. C. potential is developed at point A. Such potential cannot be transmitted through. capacitor 54. Under these conditions, then, rectiers 59 and 69 receive no input. The cathode .of diode 66 is returned to a +2 volt source to overcome contact. potential of the diode, such that in the absence of keying the grid potential of tube 56 will be zero or slightly positive with respect to ground. This insures' that tube 66 will positively energize the relay in the absence of keying.

This grid potential on grid 65 makes tube 66 of the relay. When relay 68 is energized, the lower contact thereof short-circuits the Sli-cycle input to grid 43l by connecting point C directly to ground through such contact. The circuit for this is obvious. The upper contact of relay 66 now completes a circuit as follows: -volt source, resistor 69', neon lamp' 76, ground. This will light the warning neon lamp' 16.

When relay 68 is energized, therefore, motor 4.6 will not run because the winding 45 is not energized. Thus, the tuning of oscillator 48, 49 remains hired at its most recent frequency as long 'as keying is absent. As long as keying is absent, the potential at A is steady D. C. and relay 68 remains energized. Thus, in the absence of telegraph signals at the input 6, the AFC apparatus is disabled.

For on-oif, on-off printer or telephony signals, it is` desirable to omit the limiting action as provided with FS keying. This is accomplished by throwing the three-ganged switch 9, 9d, 9b to the upper position labeled on-olf. The tube Il is then cut out of the circuit and the .5U-kc. input'is' applied at low level to the grid l2 of tube I3.. Tube i3 does not limit under this condition. If limiting were used, during on-oif space the noise would rise to a high level. We need to' distinguish between signal on and signal oif.

lf the input signal is on-off keyed at exactly 50 kc., no keying occurs at point A since such point is theny at. ground potential either with or without inputV signal. With proper tuning (no keying at point A), there is a zero potential at point B andthe tuning motor 46 is not energized. A slight error in the input frequency due to re ceiver drift will produce the saine onoffl keying voltage at A with a polarity depending on the direction of the frequency error. The time conetant` circuits 2`5-29 smooth out thisrkeying to give a nearly steady D; C; voltage at B. This will affecttube 30 to energize motor winding 4'5. Motor 46 will then rotate to tune oscillator 48, 49

in a direction to counteract the original drift.v

This will provide AFC action.

It will be seen, from the foregoing, that the tubes 23, 24 to tube 39 for all types of keying.

If the input signal is on-oi keying, even though it be at exactly 50 kc., keying appears at point D. Point D is then keyed negatively with respect to ground. With switch 9b in the on-oi position, the keyed direct potential at point .Dis' fed directly to the cathode follower driver tube 51. From there it is fed to rectiiiers 59, 68 in turn coupled to tube 66. With carrier present in the discriminator I1, I8 etc., the potential at D is negative with respect to ground. This negative potential keeps tube 66 cut off, de-energizing relay 68 and allowing the tuning motor 46 to be operated. Thus, AFC is maintained with carrier present. if the carrier disappears, the potential at point D disappears also. This brings grid 65 to a substantially zero or ground potential, causing tube 68 to conduct. The relay 68 is now energized to disable the AFC apparatus, rendering the tuning motor 46 inoperative and lighting lamp 10.

Diode clamps 1| and 12 are connected to the control grid 29 to keep the potential at this :point from going too far in either direction from ground potential. The anode of diode 1| and the cathode of diode 12 are connected directly to grid 29. The cathode of tube 1I is returned to a +2 volt source. The anode of tube 12 goes to a *2 volt source. The provision of clamps 1l and 12 was found desirable in order to not overload grid 29.

Although the tubes in Fig. y1 have all Vbeen shown as separate tubes, it will be appreciated that certain of them may be of the so-called twinelement type. For example, tubes Il and l2 may together be one tube of the BSLT-GT type. Tubes l1 and I8 may together be a GHG type tube. Tubes 2| and 51 may together be a BSN'Y-GT type tube. Tubes 23 and 24 may together be a BHG type tube; so also may tubes 59 and 69. Tubes I and 66 may together be a GSN'T-GT type tube. Tubes 3G and 3l may together be a GSL'l-GT type tube. Tubes 48 and 49 may together be a GSN'-GT type tube. Tubes 1I and 12 may together be a GHG type tube.

In Fig. 2 is disclosed a modied AFC system according to this invention. This figure illustrates, in general, an AFC circuit in conjunction with a single receiver. Actually, as will hereinafter appear, the AFC arrangement in a practical system, such as a three-set diversitysystem of description will cover operation with a single receiver, references being made from time to time, when necessary, to diversity reception. In Fig. 2, elements the same as vthose of Fig. 1 are denoted by the same reference numerals.

The transmited telegraph signals are picked up by an antenna and supplied to an RF amplifier 13 and thence to a mixer 14 where they are beaten down by signals supplied from an HF oscillator 15. The output of 14 is fed to a rst intermediate frequency amplier 19. The output of 16 is fed to a mixer 11 to which is also supplied heterodyning energy from a crystal oscillator 18. The output of 11 is fed to a second intermediate frequency amplifier 3. The output of 3 is fed to a mixer 4 to which are also supplied, by means of coaxial cable 5, oscillations from a S50-kc. oscillator to be later described.. The frequency of this S50-kc. oscillator is controlled by the AFC system of this invention. The output of 4 is fed to a third intermediate frequency amplier 19 working at 100 kc. The output of 19 is fed toav mixer 89 to which is supplied heterodyning enconstant at 10 kc. by the AFC arrangement of the invention.

As will later become apparent, the AFC system of this invention may be used when receiving either on-off or FS keyed telegraph signals. The

arrangement and 'operation for on-o' ykeyed sig-v nals will rst be described. Later, the arrangement and operation for FS keyed signals will be described.

84 indicates the secondary of a step-down transformer the primary of which is supplied by a cathode followeroutput stage of amplier 83. Opposite ends of winding 84 are connected to the separate anodes of two on-ofi detector diodes 85 and 86. The cathodes of diodes 85 and 86 are grounded. The center tap on winding 84 is the connection to the common diode load resistor 81 one end of which is grounded. Also connected to resistor 81, but not shown, are diodes, similar to '85 and 86, in each of the other two receivers of a three-set diversity system. Diversity action, by diode switching, occurs between the three receivers as the stronger signal cuts off the weaker signal diodes. This is a more or less conventional-arrangement for diversity switching.

The combined on-oi diode rectifier output appears across 81 and is D. C. coupled to a cathode follower coupling tube 88. From the cathode of 88, a connection extends through resistor 89 to the on-oli contact of a switch 90. Thence, with switch 98 in the position shown, the output signal is fed to a D. C. coupled trigger (not shown) which may control a tone keyer (not shown). The output of the tone keyer is fed to signal utilization apparatus, which might be located at a remote point. For example, if the receiver illustrated is located at Riverhead, Long Island, the signal utilization apparatus may be located in New York city.

Lead Si is `connected to an output point in amplifier 83 to feed such output to the anode of an AFC selector diode 92 the cathode 93 of which is connected through a resistor 94 to ground. Similar AFC selector diodes in the other two receivers of the three-set diversity system have their anodes fed from respective ampliers (corresponding to amplier 83) in such receivers. The cathodesl of such other AFC selector diodes are connected directly to the cathode 93 and to the upper end of resistor 94. Through rectication,v the instantaneous bias produced by the strongest signal tends to cut off the two weaker ones,thus keeping the AFC connected to the best signal at all times.

Cathode 93 is connected through a resistor to the control grid 95 of a pentode 96 and through asimilar resistor to the control grid 91 of a pentode 98. The l0 kc. signal is amplied by tubes 96 and 98 and fed through coupling condensers to the on-oi 10 kc. discriminator designated generally by numeral 99. Discriminator 99 is a somewhat modied Crosby double-tuned type unit. Discriminator 99 has two tuned circuits i Aand IDI. Circuit |08 is peaked at 9.7 kc. Circuit Iii! is peaked at 10.3 kc. The crossover is at 10 kc. Three series resistors, including a Centrally-located potentiometric resistor having anr adjustable tap H12 thereon, are connected between the anode of the upper discriminator diode 9 and the cathode of the lower discriminator diode. The cathode of the upper diode is connected to the ungrounded end of circuit |00. The anode of the lower diode is connected to the ungrounded end of circuit IOI. Tap |02 is connected to the on-o contact of a switch 90d ganged with switch 95. The ori-ofi AFC center control |02 allows adjustment of the AFC center to exactly 10 kc.

Two resistors |03 and |04 are connected in series between the anode of the upper discriminator diode and ground. A resistor |05 is connected between the cathode of the lower discriminator diode and ground. From the common point |06 of resistors |33 and |04, a connection extends to the on-ofi contact of a switch 90b ganged with switch 90.

The discriminator 09 is driven by tubes 96 and 93, the tuned circuits and I 0| driving the two rectiers. With l0 kc. input to tubes 96 and 98, both circuits drive the upper and lower diodes with equal voltages. This provides ground or zero potential at tap 3&2 and a negative potential 'at point |33. A slight change in frequency will still leave pointY I negative, buttap |02s direct potential will change in one direction or the other, according to the direction of the change in frequency. l

The oscillator to be controlled in frequency consists of a pair of tubes |01 and |08 in a cathode-coupled type oscillator having a common cathode resistor |09. A tank circuit IIU is connected between the anodes of tubes |07 and |08. The oscillatorl tuning condenser I I I, which is connected between the anode and grid of'tube |07, is coupled through a suitable gear reduction train to the shaft of a two-phase motor 46. Motor 6 is in turn controlled by discriminators and threshold circuits to be described.

The oscillator output is fed from a capacitive voltage divider, consisting of condensers ||2 and H3, to the grid Iii?- of a low impedance cathode follower coupling tube I 5. The cathode of tube I5 is connected through condenser ||3 to one conductor of coaxial line 5. In this way, the S50-kc. oscillatory energy is fed byline 5 to mixer ll, where it beats with the 450-kc. IF out of 3 to produce a U-kc. output frequency.

The oscillator including tubes |07 and |08 can also supply ocsillatory energy of 350 kc. to two other receivers (not shown) of a three-set diversity system. For this purpose, two other cathode follower coupling tubes similar to tube I|5 would be provided, one for each of the two other receivers of the diversity system. The grids of these other two coupling tubes would be connected directly in parallel to grid H0, to receive oscillatory energy from the oscillator, and a separate coaxial line similar to line 5 would 'be coupled to the cathode of each of these other two tubes, to feed oscillatory energy to a separate mixer .imilar to mixer 0 for each of the other two receiving channels of the three-set diversity system. In other words, the common oscillator feeds oscillatory energy into three separate cathode follower coupling tubes, one for each receiver of the three-set diversity system The AFC control for the tuning motor 06 is derived from the 10 kc. discriminator 99 when Vusing on-off keying. With the keying selector switch Sii, 00a, etc. in the position shown, the discriminator 90 is connected to serve as the AFC control. With switch 90a in the on-off position shown, the D. C. AFC potential at tap |02 is fed through a resistor ||`l to one grid 29 of tube l0 30, 3|. This tube `may vbe of the 12AX'7 type, a twin triode. 'Diode clamps vrIl, l2 are connected t0 the lower end of resistor lil to prevent the AEC potential applied to grid 29 from exceeding :t2 volts. Clamps 'I2 may together be a 6AL5 twin diode tube.

The other triode grid of 3G, 3| is grounded through a resistor I I8. Therefore, when the AFC voltage at tap |02 goes to zero at the crossover or center frequency of the discriminator 09, both grids of 30, 3| are at zero volts or ground potential. Across the secondary 33 of filament transformer 33, 36 are connected four series voltage divider resistors H53, |20, |2i and |22. The junction of resistors IIS and |20 is connected to cathode 32 while the junction of resistors VI2| and |22 is connected to cathode 30. The midpoint junction between resistors |20 and |2| is connected to ground through resistor 35. In this way, Sil-cycle voltages of opposite phase are sup- Dlied to the two cathodes 32 and 30. The two anodes of tube 30, 3| are connected directly together at point C. When the AFC voltage is zero, both grids of 30, 3| are at zero volts or ground potential and the S0-cycle voltages from transformer 33, 36 are amplified by both sides of 30, 3|. These voltages, being of opposite phase, produce cancellation of the fundamental voltage at the common anode point C, leaving only some unbalanced harmonics.

Point C is connected through condenser ill and resistor i2 to the grid i3 of amplifier tube One winding i5 of two-phase tuning motor fit Vis connected in the plate circuit of tube 00, one end of such winding being connected through a capacitor |23 to the plate of tube 00 and the other end of such winding being grounded. The other winding il of motor l5 is supplied from the (SO-cycle terminals 3l and 30 through a reversing switch |20 and a resistor |25.

Under the above conditions, with proper receiver tuning, zero direct potential at tap |02 and no E50-cycle voltage at C, motor winding 65 is unenergized and the'motor I3 does not rotate.

-If a frequency change now occurs due to receiver tuning drift, the AFC voltage at tap |02 and at grid 29 will vary from Zero. Let us assume rst that it varies in a positive direction. It will vary in either a positive or negative direction according to the direction of frequency drift, from l0 kc., of the discriminator input signal. Under these conditions, thel left-hand triode of 30, 3| will conduct more current and through common cathode resistor 35 will reduce the current in the right-hand triode of 30, 3|.

The 60-cycle voltage on the left triode cath- 0de will then be amplified by 30, 3| and All and be applied through condenser |23 to motor winding 05. This voltage will have a phase dinerence of degrees from the voltage in winding lil, dueto capacitor |23, and the motor 06 will therefore run. Tuning condenser will be rotated to vary the frequency of oscillator |01, 08, etc. until its frequency is such as to bring the discriminator input signal to exactly 10 kc. In other words, the motor will run until the frequency of the oscillator is such as to restore 30, 3|- to a balanced condition, with zerok voltage at tap |02 and grid 2Q.

A frequency change giving an opposite polarity voltage at |02 produces a negative voltage at grid 28, resulting in conduction of the righthand triode of 3Q, 3|. Due to the 180 phase difference of the (iO-cycle voltage applied t0 the cathodes 32 and 30, this results in an output l 1l voltage at C 180 different in phase from the former example, reversing the rotation of motor 40. This will cause rotation of in the opposite direction to bring the discriminator input frequency back to exactly kc.

The foregoing has described the operation of the AFC system for on-oft` telegraph signals, switch 90, 90a, etc. being in the on-od position.

To prevent false AFC operation on weak signals or on a signal that momentarily fades into the noise level, a threshold system is provided. This system biases off the AFC ampliiier tube 44 under these conditions. Tube |26 is the cutoff control tube. A Voltage divider, consisting of series resistors |21, |28 and |29, is connected between a +260 volt source and a 108 volt source. Resistor |21 is the plate resistor of tube |26. From the junction point of resistors |28 and |29, a connection extends through resistor |30 and resistor 42 to grid 431. Thus, at least a portion of the grid bias of tube 44 is obtained from voltage divider |21 |29, it being noted that there is a resistor |3| between the cathode of tube 44 and ground.

When tube |26 has more than about 4 volts on its control grid |32 (the cathode of tube |26 being grounded), this tube is cut oi. Then, the voltage divider |21 |29 provides normal operating bias on AFC amplifier tube 44. A neon lamp |33 is connected in series with a protective resistor |34 across resistor |21. When tube |26 is cut off, there is insufficient voltage drop across |21 to light lamp |33 and this indicator light being oil shows that the AFC circuits are normal.

Grid |32 derives its cutoff bias of about 15 volts from point |06 on the negative output side of discriminator 99, through switch 90b and resistor |35. Therefore, with normal on-oi keying, |26 is always at cutoff. Ii the signal becomes very weak, the negative voltage at discriminator point |06 drops, to make |26 conduct. When |26 conducts, thepositive voltage at the high end of resistor |28 is reduced, thus leaving a net negative bias on grid 43. This will cut off tube 44 and make the motor 46 stop. The AFC is then disabled. Under this condition, the drop across resistor |21 is sufficient to light indicator lamp |33. This indicates that the AFC is off.

With very slow on-off keying, the cutoff voltage from the discriminator 99 will alternate between volts and a lower value. This will make the indicator light |33 blink. Long time constants in the discriminator and in the cutoi control circuits minimize this condition.

The receiver of this invention has a rather large automatic gain control (AGC) range. The AGC circuits have not been shown, in order to simplify the drawings as much as possible. Due to the large AGC range of the receiver, the noise output at the discriminators tends to be high under weak signal or rio-signal conditions, resulting in the control tube |26 sometimes operating to the AFC on condition (tube |26 cut off) with noise. An additional threshold circuit is provided to squelch or disable the AFC under this condition.

A control voltage is derived from the on-of diodes 85 and 86 by means of a connection extending from the grid of tube 88 to the on-off contact of a switch 90e, ganged with switch 90. With switch 90e in the position shown, this voltage is applied through a resistor |36 to the control grid of a triode |38. Tube |38 is connected to operate as a cathode-coupled D. C. amplier.

The cathode |33 of tube |38 is connected through a series-resistor voltage divider |40, |4|, |42 to the 108 volt supply. |4| is the AFC threshold potentiometer which is set with a signal in the receiver to the point where the AFC indicator light |33 just goes out, indicating normal AFC circuits. The movable tap |43 on |4| is connected to the grid and plate of a diode-connected triode |44 the cathode of which is connected to the switch end of resistor |35. Hereafter, tube |44 will be termed a diode.

y When the tap |43 is set to a point where indicator |33 just goes out with a signal in the receiver, the diode |44 is out off (nonconductng) Then, it has no effect on the grid |32 of tube |26. A time constant circuit (not shown) at the grid of tube 68 provides AGC action for the receiver. If the on-off signal goes to 01T long enough to allow said circuit to discharge and if the accompanying rise in gain due to the AGC action produces more than 4 volts of noise at the discriminator 99, then the AFC would run on noise and detune the receiver. However, the onoii diodes 85, 86 under this condition produce at the grid of tube 86 only 10 volts of noise as against -15 volts with a normal signal.

The resulting change from -15 to -10 volts through tube |33 causes diode |44 to conduct. This clamps the grid |32 of tube |26 to a low negative potential, causing such tube to conduct. This in turn squelches or disables the AFC until the on-oi signal again is keyed or recovers sutliciently above the noise level. When this occurs, diode |44 no longer conducts and tube |26 is again cut off.

The foregoing has described the operation of the Fig. 2 system in connection with on-off keyed signals. There remains to be described the operation when receiving FSK signals.

Lead |45 is connected to an output point in amplifier 83 to feed such output to a converter unit |46. This converter or mixer is also fed oscillatory energy from an oscillator |41 and functions to convert the FSK signal from the last IF of 10 kc. down to audio frequencies to iit the mark and space filters.

The output of |46 is fed to an FSK unit |48. This unit may be of the type disclosed in my copending application, Serial No. 106,465, filed July 23, 1949, for example. Said copending application ripened on November 25, 1952 into Patent #2,619,587. It may include amplifiers, gate tubes, gate control circuits, etc., and may function to select for the output of the unit the strongest signal of a three-receiver diversity system. |49 indicates the secondary of a transformer the primary of which is supplied by a cathode follower amplier stage in unit |48. Opposite ends of winding |49 are connected to the separate anodes of two FSK system diodes |50 and 15|. The cathodes of these diodes are grounded. The center tap on winding |49 is the connection to the common FSK diode load resistor |52 one end of which is grounded. The diodes |50 and |5| aid in the diversity switching action, and may correspond to one of the diodes |4, 20 or 24 in my copending application referred to. A connection extends from the ungrounded end of resistor |52 to the FSK contact of switch 90e, for a purpose to be later described. The mark-space signal at the output of unit |48 is fed to the common inputs of a mark lter |53 and a space filter |54. The individual mark and space lter outputs are fed to separate mark and space ampliers (not shown, but as disclosed in my said application). From thence they-.are Yfed to a common rectifying and combiningcircult |55. In |55 they are rectified and combined in such a way as to produce VD. C. keying at output lead |56, for mark -3 volts to ground and vfor space +3 volts to ground. v |55 may bearranged as. disclosed in my said application.` Leadv|56 is connected to the FSKl contact of switch-98. When switch 98 is in the"FSK position, the D. C. keying at lead |55 is fed tothe D. .CL coupled trigger which controls the Ytone keyer. In this way, the received FSK signals may be utilized.

The audio outputof mark 'lter |53 is fed to the anode of an AFC mark selector diode |57 the cathode |58 of which is connected through a resistor |59 to a -bias potential of +0.23 voltand through another resistor |68 to ground. Similar AFC mark selector diodes in the other two receivers of the three-set diversity system have their anodes fed from respective mark filters (corresponding to i-llter |53) in such receivers. The cathodes of such other AFC mark selector diodes are connected directly to the cathode |53 and to the upper end of resistor |59. Through rectiicaticm the instantaneous bias produced by .the strongest signal tends `to cut oir the two weaker ones, thus keepingthe AFC connected to the best signal at all times. The action here is substantially as disclosed in-my copending application, Serial No. 154,650, filed '7, 1950, which ripened on October 7, 1952 into Patent #2,613,271.

Similarly, the audio output of space filter |54 is fed to the anode of an AFC space selector diode 6| the cathode |52 of which is connected through a resistor |63 to a bias potential of +08 volt and tothe ungrounded end of resistor |58.

Similar AFC space selector diodes in the other two receivers of the three-set diversity system have their anodes fed from respective space lters (corresponding-tolter |55) in such receivers. :i

The cathodes of such other AFC space selector diodes are connected directly to the cathode |52 and to the lower end of resistor |63. Through rectification, the instantaneous' bias produced by the strongest signal tends to'cut yoff the Vtwo 'weaker ones, thus keeping the AFC connected to the best signal at all times.v Here, again, 'the action is substantially as disclosed in my cepending application, Serial No. 154,650. Cathode |58 is connected through a coupling condenser |68 to the control grid cfa triode |65 which amplifies the mark frequency. Cathode |62 is connected through a coupling condenser |68 to the control grid of a triode |87 which amplies the space frequency. The anode of tube |65 is connected through a coupling condenser |68 to a unit |78 in order to apply the loutput of such tube thereto. The anode vof tube: |67 vis connected through a coupling condenserv |69 tounit |78. Unit- |76 consists of a mark amplifier and a space amplier for an FSK tuning indicator, preferably arranged as shownfin my copending application, Serial No. 154,650. A voltage divider, consisting of two seriesre- Lsisters |7| and |72, is connected 'between the unit |78 side of condenser |68 and ground. 'Ihe mark frequency is fed to control grid |75 of discriminator driver tube |78 by means of a connection from the junction of resistors |7| and |72 through condenser |73 and resistor |74. The mark frequency is also fed from the voltage divider to control grid |77 of another discriminator driver tube |78 through condenser`|78 and resistor |79. The mark frequency signal is ainlpliiied by tubes |75 `and |78 yand Afed..through coupling-condensers |88 and |8| to the dis-- criminator |82. Discriminator |82 -is a somewhat modified Crosby double-tuned-type unit, similar to discriminator 99. Discriminator |82 has two tuned circuits |83 and |86. Discriminator |82 has a center frequency corresponding to the audio frequency of the received mark signal which should appear, when the receiver is properly tuned, at the output of converter or mixer |46. As a typical example, discriminator |82 may have a center frequency of 2125 cycles.

Three series resistors, including a centrallylocated potentiometric resistor having an adjustable tap |85 thereon, are connected between the anode of the upperdiscriminator diode and the cathode of the lower discriminator diode. The cathode` of the upper diode is connected to condenser and to the ungrounded end of circuit |85. The anode ofthe lower diode is connected tothe condenser |8| and to the ungrounded end of circuit |85. Tap is connected to the FSK contact of switch 98a. At a predetermined center frequency, the crossover or AFC center is adjustable by 85. This enables obtaining of Zero D. C. output for a considerable range of mark frequencies about the 2125-cycle Aposition of the discriminator |82. Thus, taking into consideration the characteristics of filters |53 and |58, with the various amounts of frequency shift usable at the corresponding lter center frequencies, the AFC may be adjusted to control at any desired mark frequency.

Two resistors |85 and |87 are connected in series between the anode of the upper discriminator diode and ground, while a resistor |88 is connected between the cathode of the lower discriminator diode and ground. From the common point |89 of resistors |86 and |87 a connection extends to the FSK contactl of switch 98h.

The discriminator |82 is driven by tubes |76 and |78, the tuned circuits |83 and |85 driving the two rectiers. If the discriminator |82s 'center frequency is 2125 cycles, with a 2125-cycle niark input' to tubes |75 and |76 both circuits drive the upper and lower diodes with equal voltages. This provides zero or ground potential at 'tap |85 and a negative potential at point |89. A slight change in mark frequency will still leave point |89 negative, but tap |85s direct potential willfchange in one direction or the other, according to the direction of the change in frequency. y The 'AFC control for the tuning motor |16 is derived from the 2125-cycle discriminator |82 when using FS keying. With the keying selector switch 90, 98a, etc. in the FSK position, the discriminator |82 is connected to serve as the AFC control. With switch 98a in the FSK position, the D. C. AFC potential at tap |85 is fed through resistor ||7 to grid 29. When the AFC voltage at tap |85`goes to zero at the lcrossover or center frequency of the discriminator |82,.both grids of 36, Y8| are at ground potential. .Under these conditions, motor winding 85 is not energized and motor 46 does not rotate.

rI'he operation of the AFC, when controlled from the discriminator |82, is exactly the same as when controlled from the discriminator 99. The description of such operation therefore will not be repeated. Suiice it to say here that the frequency of the S50-kc. oscillator will be varied by motor 48 to maintain the input signal to discriminator |82 exactly at the discriminator lcenter frequency of 2125 cycles or at any predetermined nearby frequency corresponding to .the setting yof tap |85. v Y

to zero or slightly positive, causing 126 to conduct. This reduces the positive voltage at the high end of resistor 128, leaving a net negative bias on grid 43 and cutting olf 44. This will make the motor -46 stop, disabling the AFC. Also, indicator lamp 133 will bev lighted, indicating that the AFC is olf.

With switch 90, 99a, etc. in the FSK position, the additional threshold circuit including diode 144 is again effective to prevent the AFC running on noise.

A control voltage is derived from the FSK diodes 150 and 151 by means of a connection extending from the ungrounded end of resistor 152 to the FSK contact of switch 90e. With switch 90o in the FSK position, this voltage is applied through 136 to the control grid of 139. If the FSK signal goes off the air or fades to a very weak signal, the diodes 150 and 15| produce at the ungrounded end of 152 only 6.1 volts of noise as against 9.6 volts with a normal signal, since the FSK limiter follows the diode drive circuits. The resulting change from 9.6 to 6.1 volts through tube 138 causes diode 44 to conduct. This clamps grid 132 to a low negative potential, causing tube 126 to conduct. This again squelches or disables the AFC until the FSK signal again appears or recovers suiciently above the noise level. When this occurs, diode 144 no longer conducts and tube 126 is again cut off.

It should be noted that the input to driver tubes 116 and 118 of discriminator 182 is taken from mark amplifier tube 165. Therefore, as long as there is a mark signal present to pass through mark filter 153, tube 126 is cut off. The AFC is then enabled. In other words, the AFC system will continue to function if the signal stops on mark, a condition commonly occurring with printer operation. The circuit of Fig. 1 does not have this feature.

As previously explained, with FSK and AFC system of Fig. 2 is disabled if the signal stops on space or stops keying. With FSK, the AFC system of Fig. 1 is disabled only when keying stops.

Although in Fig. 2 the tubes have been illustrated in most cases as separate, it is to be understood that this is not absolutely essential. Pairs of electrode structures of Fig. 2 may be combined in single envelopes, in tubes of the socalled twin-element type. Examples of the latter which may be used are: 6AL5, 12AU7 and 12AX'1.

Fig. 3 discloses a modified AFC motor control circuit. This ligure is somewhat simplied as compared to Figs. l and 2. In Fig. 3, elements the same as those of Figs. l and 2 are denoted by the same reference numerals. The direct current output of a discriminator or other suitable source is applied as input to the terminals 190 and 191 of the AFC system. Terminal 191 1s grounded. The direct potential at terminal 190 may be either plus or minus with respect to ground 191, depending on the direction of variation of the discriminator ,input frequency with '16 respect Ato a predetermined desired frequency. The magnitude of such potential at 190 depends upon the amount of frequency error. This is the well-known operation of a discriminator.

Terminal 190 is connected through a resistor 192 to point F andthe anode of a diode 193 and to the cathode of a diode 194. In this way, potentials at 190 are applied to the anode of 193 and to the cathode of 194. Diodes 193 and 194 are also fed with alternating current. This is effected by a connection from the cathode of |93 through resistor 195 to one end of the secondary 196 of alternating current input transformer 191 and by a connection from the anode of 194 through a resistor 198 to the other end of secondary 196. Two equal resistors 199 and 200 are connected in series between the cathode of 193 and the anode of 194. The common junction of these two resistors is grounded. The primary 201 of transformer 191 is connected to a suitable source of alternating current, such as the ordinary ll5-volt, 60-cycle source. Resistor 192 is very large as compared to resistor 195, 199, |99 and 200.

Point F is connected through a coupling condenser 202 to the control grid of the first tube 203 of a two-stage condenser-coupled amplifier denoted by numeral 204. Amplifier 204 is a more or less conventional voltage amplifier, amplifying the voltages appearing at point F. The anode output of the second-stage tube 205 is coupled through a phasing condenser 123 to one of the motor windings 45 of a two-phase motor 46. Winding 45 may be described as the controlled motor winding. A capacitor 206 is connected across winding 45 to tune the same to resonance at 60 cycles, thus providing a parallel resonant circuit in the output of amplifier 204.

T'he other winding 41 or motor 46 is connected through a resistor to the same source of alternating current used to supply primary winding 201. Winding 41 is therefore constantly energized. The shaft of motor 46 drives the tuning condenser of a heterodyne oscillator (not shown) which governs the frequency of the input to the same discriminator to the output side of which terminals and 191 are connected.

First assume that there is zero direct current potential at terminals 190 and 191. In other words, assume that terminal 190 is also at ground potential. There would =be zero direct potential at the output of the discriminator, to which terminals 190 and 19| are connected, when the receiver is properly tuned. Under these conditions, motor 46 should be unenergized. During the half of the A. C. cycle in which the righthand end of 196 is positive and the left-hand end thereof negative, diodes |93 and 194 both conduct in series (through resistors and 198). The diodes are so poled that they will conduct in series during this half-cycle. During the other half of the A. C. cycle, in which the right-hand end of |96 is negative and the left-hand end ,thereof positive, the diodes 193 and 194 are both nonconducting. The diodes are so poled that they are nonconducting during this other halfcycle.

During the periods of diode conduction, point F is clamped to ground, since the resistors 195 and 198 are equal and since in effect the center of winding 196 is grounded (resistors 199 and 200 being equal). During the periods of diode nonconduction, point FY immediately assumes the voltage at the input terminal 190, since there is then no load from F to ground. Point F then With zero'D. C. input at terminals i90-49H then, point F remains steadily at zero or ground and no A. C. is passed to amplifier 204. Motor winding 45 is unenergized under these conditions, such winding being supplied only by amplifier 204s output. The receiver is then properly tuned and the AFC is inactive.

Since resistor 92 is a great deal larger than resistors 95 and 98, a small D. C. voltage at the input i90-ISI cannot change the voltage at F by an appreciable amount, during the periods of diode conduction. Point F is, therefore, clamped to ground during one-half of each complete cycle of the 60-cycle drive, regardess of the D. C. input potential. Therefore, even when the receiver is improperly tuned, when there is an appreciable direct potential at the receivers discriminators output, during periods of diode conduction point F is at ground or zero potential.

As previously described, point F immediately assumes the voltage at the input terminals during the periods of diode nonconduction. Therefore, with a positive D. C. input voltage at i90-lill, point F will alternate between this positive D. C. voltage and ground at a {S-cycle rate. With a negative D. C. input voltage, point F will alternate this negative D. C. voltage and ground at a 60-cycle rate.

Amplifier 200 is capacitively coupled (at 202) to the diode chopper |93, 194 etc. Hence, only the (iO-cycle A. C. component is amplified and fed to motor winding 05. Therefore, when the receiver is improperly tuned, there is a direct potential at i90-49| and 60-cycle A. C. is fed to motor winding 45. Winding 41 being continuously supplied with 60-cycle A. C. from the line, motor i5 will now be energized and will rotate to change the receivers tuning. Therefore, the AFC will be active to properly tune the receiver.

With a positive D. C. input at H10-|91, point F alternates between this positive voltage and ground; with a negative D. C. input, point F alternates between this negative voltage and ground. Therefore, the phase of the A. C. applied to the grid of tube 203 reverses when the D. C. input potential goes from positive to negative. The verity of this statement should be apparent to those skilled in the art. Hence, motor 46 will run one way with positive D. C. input at i90-l 9i and the other Way with negative D. C. input. The AFC action of the tuning motor arrangement of Fig. 3 will therefore be proper.

The voltage excursion of F relative to ground cannot exceed one-half the peak-to-peak value of the transformer A. C. voltage, due to the clamplng action of the diodes. Thus, when F tends to go too far negative relative to the negative voltage on the anode of diode I 94 (this negative voltage on such anode being present during periods of diode nonconduction), this diode will conduct to clamp F to ground. When F tends to go too far positive, diode |93 will conduct to clamp F to ground. This action has the effect of limiting the maximum A. C. voltage that can be applied to the input of amplifier 204. This limiting is desirable, to prevent overload. In other words, during half-cycles of the A. C.) of diode nonconduction, point F assumes the D. C. potential of the input, as long as this D. C. potential does not exceed the peak value of the A. C. applied to the diodes.

The circuit of Fig. 3, including the 60-cycle resonant circuit 45, 206, is arranged toimprove the wave form of the rather square waves or pulses applied to the amplier 204. It also provides (due in part to capacitor |23 and to the choke in the plate lead of tube 205) the required phase relation between the voltages appliedto the two motor windings 45 and 41.

What I claim to be my invention is as follows: l. An automatic frequency control system, cornprising means, to which signals are fed, for uti--A lizing such signals, tunable means for controlling the frequency of the signals fed to said utilizing means, a two-phase motor having two field windings and having an armature coupled to said tunable means to vary the tuning thereof, a source of alternating current coupled vto one eld winding, a pair of electron -discharge kdevices having input, output and control electrodes, connections for applying alternating current from said source antiphasally to corresponding input electrodes of each of said devices, a connection between the output electrodes of both said devices and the other field winding, means coupled to said utilizing means for deriving a potential which varies with respect to a reference value in response to variations of the frequency of said signals from a predetermined value, and means for applying said potential to the control electrode of one of said devices.

2. An automatic frequency control system, comprising an amplier to which signals are fed, a tunable heterodyne oscillator for controlling the frequency of the signals fed to said amplifier, a two-phase motor having two field windings and having an armature coupled to said oscillator to vary the tuning thereof, a source of alternating current coupled to one field winding, a pair of electron discharge devices having input, output and control electrodes, connections for applying alternating current from said source antiphasalto corresponding input electrodes of each of said devices, a connection between the output electrodes of both said devices and the other field winding, means coupled to said amplier for deriving a potential which varies with respect to a reference value in response to variations of the frequency of said signals from a predetermined value, and means for applying said potential to the control electrode of one of said devices.

3. An automatic tuner for frequency shift keyed or on-off keyed telegraphy, comprising means, to which telegraph signals are fed, for utilizing such signals, tunable means for controlling the frequency of the signals fed to said utilizing means, a frequency discriminator detector coupled to said utilizing means, said discriminator detector including circuits tuned respectively above and below the desired signal fren quency and rectiers coupled in opposed relation by rectifier loads, apparatus for tuning said tunable means, connections to said loads for deriving the difference of the potentials developed across such loads, means for controllingsaid apparatus by said difference potential, and means connected to said loads and responsive to the absence of at least one of the potentials developed across such load-s for disabling said apparatus in the absence of 'keyed signals.

4. An automatic tuner for frequency shift keyed or on-off keyed telegraphy, comprising means, to which telegraph signals are fed, for utilizing such signals, tunable means for controlling the frequency of the signals fed to said utilizing means, a frequency discriminator detector coupled to said utilizing means, said discriminator detector including circuits tuned respectively above andV below the desired signal frequency and rectiiiers coupled in opposed relation byrectiier loads, apparatus for tuning said tunable meas, connections to said loads for deriving the dierence of the potentials developed across such loads, means for controlling said apparatus by said difference potential, a'controllable current conducting path connected in Vseries with said apparatus, and means coupled to said loads and responsive to theabsence of at least one of the potentials developed across such loads for controlling said path to disable said apparatus in the absence of keyed signals.

5. An automatic tuner for frequency ,shift keyedor on-oif keyed telegraphy, comprising an amplifier to which telegraph signals are fed, a tunable heterodyne oscillator for controlling the frequency of the signals fed to said amplifier, a freqeuncy discriminator detector coupled to'said amplifier, said discriminator detector including circuits tuned respectively above and below the desired' signal frequency and rectiiiers coupled in opposed relation by rectifier loads, apparatus including a controllable tuning motor for varying the tuning 'of said oscillator, conections to said loads for deriving the difference of the potentials developed across such loads, means for controlling said motor by said difference potential, and means connected to said loads and responsive to the absence of at least one of the potentials developed across such loads for disabling said apparatus in the absence of keyed signals.

6. An automatic' tuner for frequency shift keyed or on-oif keyed telegraph signals, comprising means, to which telegraph signals are fed, for utilizing such signals, tunable means for controlling the frequency of the signals fed to said utilizing means, a frequency discriminator detector coupled to said utilizing means, said discriminator detector including circuits tuned respectively above and below the desired signal frequency and retifiers coupled in opposed relation by rectifier loads, apparatus for tuning said tunable means,l said apparatus being coupled for energization thereof to a source of alternating current, connections to said loads for deriving the difference of the potentials developed across such loads, means for controlling` said apparatus by said difference potential, a controllable amplifier device between said source and said apparatus, means coupled to said loads and responsive to the absence of at least one of the potentials developed across such loads for cutting off said device to disable said apparatus in the absence of keyed signals, and additional means, responsive to the presence in said utilizing means of signals below a predetermined strength, for cutting off said device to disable said apparatus.

7. An automatic tuner for frequency shift keyed or on-off keyed telegraph signals, comprising an amplier to which telegraph signals are fed, a tunable Vheterodyne oscillator for controlling the frequency of the signals fed to said amplifier, a frequency discriminator detector coupled to said amplier, said discriminator detector including circuits tuned respectively above and below the Adesired signal frequency and rectiers coupled in opposed relation by rectifier loads, apparatus for varying the tuning of said oscillator, said apparatus being coupled for energization thereof to a source of alternating current, connections to said loads for deriving the difference of the potentials developed across such loads, means for controlling said apparatus by said difference potential, a controllable ampli-v fier device between said source and said apparatus, means coupled to said loads and responsive to the absence of at least one of the potentials developed across such loads for cutting off said telegraph signals are fed, for utilizing such signals, tunable means for controlling the fre-` quency of the signals fed to said utilizing means,

means coupled to said utilizing means for deriving a potential which varies with respect to a reference value in response to variations ofthe mean frequency of said signals from a predetermined value, apparatus controlled by said potential for tuning said tunable means to change the means frequency of said signals back to said predetermined value, and means responsive to a potential at the output of said potential deriving means for disabling said apparatus if keying of the telegraph signal stops on a predetermined one of said two signal frequencies,

9. A system in accordance with claim 8, wherein the predeterminedone of the two signal frequencies is that representing space.

.10. In an automatic control system for radio apparatus having a mechanically-driven control.

member, a two-phase motor having two field windings and having an armature mechanically coupled to said member to drive the same, a source of alternating current coupled to one eld winding, a pair of electron discharge devices having input, output and control electrodes, connections for applying alternating current from said source antiphasaly to corresponding input electrodes of each of said devices, a connection between the output electrodes of both said devices and the other field winding, and means for varying the potential on the control electrode of one of said devices.

BERTRAM A. TREVOR,

RefeYenScitsd mule nie or this patent UNITED STATES PATENTS Semin Sept. 11, 

